Thinking Books: Terry Dear's Classified
These aren't sf but I can see them appealing to kids who like to puzzle things out. Each fictionalizes a real life mystery/case history, and then takes the reader through various possibilities. I rather enjoyed them.
All were reprinted by Kingfisher in 2004.
Break Out (1996): investigation into the disappearance of an imprisoned spy.
Discovery at Roswell (1996): young FBI man investigates Roswell.
Vanished! (1996): Investigation into the Philadelphia Experiment.
The Nuclear Winter Man (1996)--investigation into the disappearance of a Russian scientist.